Television background control system



Jan. 13, 1948. c. E. HALLMARK 2,434,405

TELEVI S ION BACKGROUND CONTROL SYS TEM Filed June 29, 1944 TRANSMITTER INVENTOR CLYDE E. HALLMARK ATTORNEY Patented Jan. 13, 1948 TELEVISION BACKGROUND CONTROL SYSTEM Clyde E. Hallmark, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application June 29, 1944, Serial No. 542,776

Claims. (01. 179-171) This invention relates to television systems and particularly to apparatus employed in such systems for effecting background control.

In order to generate television signals suitable for use in reproducing a television subject satis- I factorily, it is necessary that the signals convey, not only the video information representative of the variations of light and shade of the subject, but also information regarding the average subject illumination. The latter type of information generally is referred to as background information or the direct current or low frequency component of the television signal. The television signals derived from any of the image analyzing tubes employed for their generation require considerable amplification to render them suitable for transmission. By reason of the relatively high frequency of the video signals and also of the width of the frequency band required for the transmission of such signals, it frequently is preferable to employ a video signal amplifier comprise ing a plurality of stages, the coupling between stages being eifected by means of condensers.

As is well known in the art, however, capacitatively coupled amplifiers will eifect the amplification only of the relatively high frequency video signals. The relatively low frequency direct current component, even though present in the signals derived from certain types of image analyzing tubes, therefore, is lost in the video signal amplifier. In order that the background information signals be transmitted along with the video signals, it is necessary that the direct current component be supplied to the transmitter apparatus prior to the signal modulation of the carrier wave which is to act as the transmission medium.

It is an object of the present invention, therefore, to provide television apparatus arranged in a novel manner to effect the insertion of the direct current signal component into the video signals.

In accordance with the present invention, there is provided a source of television signals which comprise a video component representative of the light and shade values of a television subject and a direct current component representative of the average subject illumination. Thi source includes an electronic device having a plurality of electrodes. The video signal component of the television signal is amplified by means reactively coupled to one of the electrodes. A unidirectional voltage corresponding to the direct current component of the television signal is developed by means coupled to another of the electrodes. Ad

ditionally, there is provided means for impressing the unidirectional voltage upon the input circuit of the video component amplifying means.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing the single figure is a diagrammatic representation of television signal generating, amplifying and transmitting apparatus.

Having reference now to the drawing, the invention is disclosed in an embodiment employing an image dissector type of analyzing tube. Only the components of such a tube necessary for an understanding of the present invention have been illustrated for the reason that tubes of this character are well known in the art. Also, in order not to complicate the drawing unnecessarily, the tube elements have been shown only in their approximate locations and the relative sizes of the components are not in true proportion. The tube includes a photosensitive cathode I located close to one end and a metallic anode structure 2 located close to the opposite end of the tube. The anode is provided with a small recess 3 which faces the sensitized surface of the cathode I. A primary scanning aperture 4 is formed in the rear wall of the anode recess. The scanning aperture preferably is in substantial alignment with the center of the cathode.

There is mounted within the anode 2 an electronic device having a plurality of electrodes uch as, for example, a multistage electron multiplier. The multiplier comprises a plurality of secondary electron emissive electrodes such as 5 and 6. As illustrated, these electrodes are conventional box type structures. They may be provided in any desired number, depending upon the amplification required and may be arranged in any desired manner. The first stage electrode 5 is provided with a secondary scanning aperture 1 which is aligned with the primary aperture 4 and the center of the cathode l. The multiplier also is provided adjacent the final multiplying stage with an electron collecting electrode 8.

The apparatus described is energized by suitable connections made to a voltage divider 9. The voltage divider comprises a series connection of a plurality of resistors across the terminals of which is connected a source of unidirectional energy such asa battery I I. The terminal of the voltage divider which is connected to the negative terminal of the battery II also is connected to the cathode I. The anode 2 and the first stage multiplier electrode 5 are connected together internally of the anode and also to a more positive point on the voltage divider 9 than that to which the cathode I is connected. In like manner, the other multiplier electrodes are connected to successively more positive potential points of the voltage divider. The collector electrode 8 is con nected through a load impedance device, suchas a resistor I2, to the grounded positive terminal of the voltage divider 9 and the battery II.

The image analyzing tube also is provided with electromagnetic deflecting coils I3 and I4 which, Wnen suitably energized, effect horizontal and vertical deflection of the electron image produced by the cathode I. The coils I3 and I4 are energized respectively by substantially sawtooth voltage sources I5 and I6.

The video signal amplifier includes vacuum tubes Il, I8 and I9. Each of these tubes is illustrated as a triode but it is contemplated that other types of tubes, such as multigrid tubes, usually used in amplifiers of this type may be employed in accordance with this invention with equal facility. The control grid of the first stage amplifier tube I1 is conductively coupled to the ungrounded terminal of the multiplier load resistor I2. The cathode of this tube is connected to ground through a resistor 20. The anode of the tube I! is connected through 'a load resistor ,2I and an inductor 22, to the positive terminal of a source of unidirectional energy such as a battery 23. The negative terminal of this battery is grounded. The anode of the tube Il also is reactively coupled by a reactance device such as a condenser 24 to the control grid of the secnd stage amplifier tube I8. A leak resistor 25 is connected between the control grid of the tube I8 and ground. The cathode of the tube I8 also is connected to ground through a resistor 26. The anode of this tube is connected through a load resistor 21 and an inductor 28 to the positive terminal of the battery 23.

The anode of the tube I8 is reactively coupled by a reactance device such as a condenser 29to the control grid of the final video amplifier tube I9 which, in this case, also is a signal combining tube. A leak circuit for the coupling condenser also is provided by the series connection of a fixed resistor 31 and an adjustable resistor 32, the latter of which is connected to ground. The cathode of the combining video amplifier tube I9 is connected through a cathode follower resistor 33 to the negative terminal of a source of unidirectional energy, such as a battery 34. The positive terminal of this battery is grounded. The anode of the tube I9 is connected directly to the positive terminal of the battery 23. A transmitter 35 is connected to the cathode terininal of the resistor 33 and also to ground.

The secondary electron emissive electrode 6 of the multiplier is connected to a point on the voltage divider 9 through a resistor 36. This resistor is shunted by a bypass condenser 31. An auxiliary amplifier tube 38 is provided to amplify the direct current component of the generatedtelevision signal. The control grid of this tube is connected to the multiplier electrode 6 and the cathode thereof is connected to a point on the voltage divider 9 which is slightly more positive than the point to which the multiplier electrode Bis connected. The anode of the -tube,38 is connected to ground through the adjustable resistor tively in intensity directly in accordance with the illumination of the elemental areas of the oathode dem which the streams emanate. As is wen known in the art, a device of the character "electrodes 'is the battery "I I.

disclosed herein for generating television signals is capable of producing such signals containing both the video and the direct current information. After multiplication of the electron streams produced by the scanning of the electron image, 't'heresultingelectrons are collected by the electrode 8 and effect a current flow through the load resistor I2. The variations in magnitude of the'current flowing through the load resistor are in accordance with the video and direct current information. Consequently, corresponding voltages 'are developed in the load resistor.

These voltages are impressed upon the input circuit of the first -stage, including the tube I1, of the video signal amplifier. As a consequence, amplified signal effects, including both the video and direct current information, are developed in the output circuit of the 'tube I'I. However, by reason of the employment of the coupling condenser 2'4 between the output circuit of'the tube and the input circuit of the tube I8, only the relatively high frequency components of the signal effects are impressed upon the input circuit of the tube I 8. The relatively low frequency components comprising the direct current infor- 'mation are not transmitted by the condenser 24. Therefore, the still further amplified signal effects developed in the outputcircuit of the video amplifier tube '-I-8 constitutes only 'the video information.

The impingement of the primary electrons, derived from preceding'stag'es of the multiplier, upon the secondaryem issive surface of the electrode 6 causes the emission therefrom of an increased number of secondary electrons. The source 'of supply for the additional electrons emitted by this andall others of the multiplying In the case of the electrode fijthe supply circuit includes the resistor 36 in which 'ther'eis developeda voltage difference by reason of the electron current flow therethrough. The magnitude of the voltage developed across the r'esji'st'or'llfi varies in accordance with variationsin the' nuinb'er of primaryelectrons impinging upon the ele'c'trod'efi. "I hese, in turn, vary in accordance with the light projected upon different elemental areas of the cathode I. The result is that t'he potential variations occurring at the multiplier' electrode -'6 are in accor 'ance with the television"signalls'and include both the video and the direct current infoima'tion. However, for the present pu'r-pos e" only those voltage variations, which are representative "of the direct current information are needed. Accord ingly, the provision o'f th'e'bypas's condenser 3! shunting the resistor 38 "aifords 'ai relat'ively low impedance path for the relatively high'freduen'cy components constituting the video information. In'this manner the voltage of the multiplier electrode 6 is maintained substantially constant with respect to the video components but is caused to vary slightly with respect to and in accordance with the direct current information.

The cathode of the auxiliary amplifying tube 38 is connected to a point on the voltage divider 9 which is normally of the order of 5 volts positive with respect to the point to which the resistor 38 is connected. By this means the control grid of the tube as is biased at a predetermined negative value with respect to the cathode. Variations of the voltage occurring at the multiplier electrode 6 in accordance with the direct current information thus effect corresponding variations in the magnitude of the space current conducted by the tube 38. Inasmuch as the space current for this tube also flows through the adjustable resistor 32, there is effected a voltage variation at the anode of the tube 33 which is dependent upon the voltage variation of the multiplier electrode 5.

This voltage variation is impressed upon the control grid of the signal combining amplifier tube 59 and acts as a variable bias for the input circuit of this tube. Consequently, it is seen that relatively rapid fluctuations of the space current in the tube [9 are effected by the "ideo signal voltages impressed upon the control grid by the coupling condenser 29. At the same time relatively slow fluctuations of the space current in this tube are effected under the control of the direct current signals impressed upon the control grid of this tube by means of the resistors 3i and 32. As a consequence, there is developed in the cathode follower resistor 33 a television signal which includes both the video and the direct current information.

In a transmission system wherein negative polarity of modulation is employed (i. e., minimum carrier amplitude represents maximum light intensity and vice versa), the video signal amplifier, exclusive of the signal combining stage, may include any even number of stages. This requirement is easily understood when it is considered that the intensity of an electron stream admitted to the multiplier is directly proportional to the illumination intensity of the elemental area of the cathode i from which the stream emanates. The result is that the greater the light intensity upon an elemental area of the cathode, the greater will be the number of electrons collected by the electrode 8. Inasmuch as electronic energy is collected from the multiplier by this electrode, the greater the number of electrons collected thereby the more negative will be the potential of this electrode. In other words, the positive potential of the collector electrode 8 varies inversely to light variations of elemental areas of the television subject. When these potentials are impressed upon the control grid of the tube 1 l the conduction of space current therein varies in magnitude inversely to the variations of light intensity on the elemental areas of the cathode i. As a consequence, the voltage variations at the anode of the tube ll vary directly to the light variations. Similarly, these directly varying voltages are converted by the tube I8 Thus, by-

6 the transmitter 35 fromthe cathode follower resistor 33, these signal efiects also vary inversely .to the light intensity variations, which is the desired result. Also, it is to be understood that the transmitter 35 is of suitable character to effect the desired carrier modulation in response to the described signal effects impressed thereon.

Any odd number of auxiliary amplifier tubes, such as the tube 38, may be employed to amplify the direct current component of the television signal. The reason for this is that the control voltages for the tube 38 are derived from a multiplier electrode, such as 6, rather than from a collector electrode such as 8. In response to an increase of the light intensity on an elemental area of the cathode i, a correspondingly increased number of electrons are admitted to the multiplier. As a consequence, an increased number of electrons impinge upon the secondary emissive surface of the electrode 6 to release a correspondingly increased number of secondary electrons therefrom. The excess of these electrons over the impinging electrons represents a net loss in electrons by the electrode 6 and, consequently, serves to increase positively the potential of the electrode 5 in response to an increase of the light intensity. In other Words, the variation of the voltage impressed upon the input circuit of the tube 38 is in direct proportion to the variation of light intensity. Consequently, in the output circuit of the tube 38, the variation of the anode voltage is inversely proportional to the light intensity variation. It, therefore, is seen that the voltage variation representative of the direct current component is of suitable polarity for impression upon the control grid of the signal combining tube 19 to effect'the desired voltage variation in the cathode follower resistor 33.

It is contemplated to be within the scope of the invention that the direct current component, instead of being derived from the multiplier electrode 6, may be derived from any other multiplier electrode. Also, the video component amplifier may be coupled to one of the multiplier electrodes, and the unidirectional voltage corresponding to the direct current component may be derived from the collector electrode 8. In such a case the relative magnitudes of the two television signal components may be such that no additional direct current amplification will be needed. Furthermore, it also is deemed to be Within the scope of this invention to derive the respective video and direct current components from differen multiplier electrodes.

It is apparent that, by use of the described apparatus, a more efiicient utilization of the tube complement of television signal generating apparatus is effected. The use of certain of the electron multiplier electrodes required for the electron multiplication in the additional capacity of furnishing amplifier control voltages representative of the direct current component of the tele- Vision signal enables the elimination of additional equipment required for this purpose in arrangements of the prior art.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

' i itot 7 What is claimed is: 1. In a television 'systema source of television signals comprising avideo component representative of 'lightand shade values of atelevision subj'ec'tand a direct current component represent'ati've "of average subject illumination, said source including an electron multiplier having a plurality bf electrodes comprising a'secondaryelectron kmis'sive electrode and an electron collector ele'ctrade, a video component amplifier having an input'circuit, means including a reactance device fioricoupling said input circuit to one of said elecftrodes, and a direct current component amplifier "having its input circuit coupled to another of 'saidelectrodes and its output circuit connected fto the input'circuit of said video component am- -plifi'er.

'2. In a 'televisions'ystem, a source of television "fsigri'al's comprising a video component representative of light andshade values of a television subject and a direct current component repre- 'sentative of average subject illumination, said "sourceincluding an electron multiplier comprisingas'econd'aryelectron emissive electrode and an electroncollectorelectrode, an alternating cur- "rent "amplifier coupled to said collector elect rode, andadirect current amplifier having its in- "put circuit-coupled to said secondary electron emissive electrode and its output circuit connecreato the input circuit of said alternating -3. In atelevision system, a source of television signals comprising a video component represubjecvand' a direct current component repre- -sentative of average subject illumination, said 'source' includi'ng anelectron multiplier comprising a secondary electron emissive electrode and an electron collector electrode, a video component amplifier comprising a vacuum tube, means ineluding a'c'ondenser for coupling said tube to said -collectorelectrode, an auxiliary vacuum tube am- -=plifier having its'input circuitcoupled to said secondary' electron emissive electrode and its output circuit connected to the-input circuit of said video -amplifier "vacuum tube, and means connected to the input circuitof "said auxiliary vacuum tube amplifier-for bypassing said video component.

4. -In a television system, a source of television signals comprising'a video component representativeof light-and shade values of a television subject and-a direct'current component representative of average subject illumination, "said source including an electron-multiplier compris- *sen'tative of light and'shadevalu'es of a television 73,? a)

'ing a plurality "of secondary electron emissive electrodes and an electron collector electrode, a

video component amplifier comprising a plurality of vacuum tubes, a first one of said tube being coupled conductively to said collector electrode andthe remaining tubes being coupled capacitatively to=one another, an auxiliary vacuum tube amplifier having its input circuit coupled to one of said secondary electron emissive electrodes and "its output circuit connected to the input circuit 'of a secondone of said video component amplifier tubes, and "means including a condenser con- -necte'd in shunt with the input circuit of said auxiliary vacuum tube amplifier for bypassing'said video component.

'5. In 'a television-system, a source of television signals comprising a video component representative of light and shade values of a television subject and a direct current component repre- "sentative of average subject illumination, said source including a multistage electron multiplier comprising a plurality of econdary electron emissive electrodes and an electron collector electrode, a multistage video component amplifier comprising a plurality of vacuum tubes, a first one of said tubes being coupled conductively to said collector electrode and the remaining tubes being coupled capacitatively to one another, an auxiliary vacuum tub 'amplifier having its input circuit conductively coupled to one of said secondary electron emissive electrodes and its output circuit connected to the input circuit of a second one of said video component amplifier tubes, and a resistive-capacitative networkcon- 'nected in 'shuntwith the input circuit of said auxiliary vacuum tube amplifier,

CLYDE E. HALLMARK.

REFERENCES CITED The following references areof record in the file of thispatent:

UNITED STATES PATENTS 

